The present invention relates to a tear-off cap or shearable screw.
For connecting, branching off and plugging of cables, in particular underground cables, screw connectors, cable connecting clamps or cable lugs are conventionally used, into which tear-off cap screws are inserted for fixing or contacting of the conductors, respectively.
WO 96/31706 discloses a shearable screw comprising a plurality of thread portions and a cap portion connected with the thread portions by means of a predetermined breaking point, wherein further predetermined breaking points in the form of cross-section tapering are provided between the thread portions. The predetermined breaking points are configured such that the predetermined breaking point being most remote from the cap features the largest cross-section tapering, whereas the point being closest to the cap features the slightest cross-section tapering or cutting depth. Due to this configuration, a tear-off torque results which always increases from the thread portion that is most remote from the cap portion towards the cap portion.
A plurality of disadvantages are, however, entailed therewith. The less the shearable screw penetrates the cable connector to clamp the conductor inserted therein, the less is the corresponding tear-off torque, i.e. correspondingly less is the force acting thereagainst. This means that the least pressing force is available for the largest conductors and the strongest pressing force is available for the smallest conductors. In the one case, namely that of the least pressing force, there is the risk that the conductor will not be clamped sufficiently and thus the danger of lacking contact, whereas in the other extreme case, i.e. the smallest conductor with strongest pressing force, a squeezing of the smallest conductor may be possible, which may result in the failure of the cable connection.
A similar shearable screw is known from EP 0 692 643 A1, where the tear-off torques of all predetermined breaking points are to increase or decrease from the cap portion towards the last or front thread portion, respectively. Possibly they are, however, substantially equal. In the case of decreasing, the afore-mentioned disadvantages result. In the case of increasing, there is the risk of the tear-off cap screw to tear off before the tear-off torque required for the respective conductor has been reached, and this does not only result in a possibly insufficient clamping of the conductor, but also entails a very disadvantageous projecting of the broken remaining portion of the tear-off cap screw from the outer contour of the cable clamp.